Stents and other radially expandable endoprostheses are typically implanted transluminally and enlarged radially after being introduced percutaneously. Such endoprostheses may be implanted in a variety of body lumens or vessels such as within the vascular system, urinary tracts, bile ducts, fallopian tubes, coronary vessels, secondary vessels, etc. Some may be used to reinforce body vessels and/or to prevent restenosis following angioplasty in the vascular system. They may be self-expanding, expanded by an internal radial force, such as when mounted on a balloon, or a combination of self-expanding and balloon expandable (hybrid expandable).
Within the vasculature it is not uncommon for stenoses to form at a vessel bifurcation. A bifurcation is an area of the vasculature or other portion of the body where a first component vessel divides into two or more component vessels. Where a stenotic lesion or lesions form at such a bifurcation, the lesion(s) can affect one, two or all three of the involved vessels.
Many of the stents that have been disclosed for deployment at bifurcations are deployed as a first stent, extending from one component vessel into a second, crossing the vessel opening (“ostium) into the third vesssel. After the stent has been deployed, an opening in the stent side-wall disposed at the ostium can then be enlarged by placing a balloon therethrough and expanding the balloon. This opening enlargement facilitates fluid flow into or from the third vessel. If needed, a second stent may also be placed in the third vessel. For a variation of this procedure, many stent configurations have been designed which have a specialized side-branch opening through which the opening into the third vessel may be provided. Often such designs include a portion of the first stent which is displaced into and against the side-wall of the third vessel for a short distance beyond the ostium.
In such stent placement procedures the balloon used to provide enlargement of the side-wall opening may be a conventional cylindrical balloon, or a stepped balloon having two cylindrical portions the distal portion typically having a smaller diameter than the proximal portion. However, use of these balloons has not always been fully satisfactory. For instance, in some applications the side-wall opening enlargement has been observed to cause an inward displacement of a portion of the stent into the second vessel flow channel, thereby potentially facilitating restenosis or otherwise disrupting the flow in the second vessel.